Additive manufacturing techniques such as 3D printing are rapidly being adopted as useful techniques for a host of different applications, including rapid prototyping and the fabrication of specialty components. To date, most additive manufacturing processes have utilized polymeric materials, which are melted, layer-by-layer, into specified patterns to form 3D objects. The additive manufacturing of metallic objects presented additional challenges, but techniques have been more recently developed to address these challenges.
Existing technologies for the additive manufacture of metal structures may generally be classified in three categories: laser sintering, adhesive bonding followed by sintering, and molten metal deposition. The two sintering technologies use a bed of metal powder in the build area, and the powder particles are selectively joined to one another to form the desired pattern. When one layer is completed, more metal powder is spread over the first layer, and powder particles are joined to the previous layer in the pattern required for that layer. The process continues with fresh powder spread over the entire surface of the build area and then selectively joined, building the desired structure layer by layer. The finished part is retrieved from inside the powder bed, and the powder is then emptied from the build area to begin the next part.
However, the use of metal powder as a raw material can be problematic for several reasons. Metal powder is expensive to produce, and generally is more expensive than a wire made from the same material for the same volume of material. Metal powders are difficult and dangerous to handle. For example, metal powder that is spilled may form dust in the air that is dangerous to inhale, and such dust may even be an explosion risk. In addition, the amount of powder required for conventional additive manufacturing technologies is many times greater than that required to make the part, as the entire build area must be filled with powder. This increases the cost of the process and leads to attrition and waste of powder, which may not be readily reused. Conventional powder-based processes are also very slow because the spreading of concurrent layers of powder typically must be done precisely to the required layer thickness and must be done across the entire build area for each layer.
Laser sintering uses a high power laser as the source of heat to fuse particles. Lasers have many safety risks, especially at the powers required for fusing metals. Using lasers as a source of heat causes issues because the particles must be heated top down to add enough heat to fuse them to the previous layer. Such top-down heating requires much more heat than would be needed if the heat was applied directly to the joining surfaces, which slows down the overall process and causes the excess heat to be dissipated into the powder bed. Because of this, there is the danger of unwanted sintering particles in the area around that which the laser is heating. Therefore, the process requires the use of metals and alloys that have poor heat conduction.
Adhesive bonding uses glue to join adjacent powder particles instead of directly fusing the particles by laser energy, but the process is otherwise similar. Glue is selectively sprayed to form a pattern, and powder is added layer by layer to form the structure. To make a mechanically sound metal part, the structure generally must be removed from the powder bed and placed in a furnace to sinter the bonded metal powders. The sintering multiplies the complexity of the process and well as the time required to produce parts.
In molten-metal deposition techniques, heat to liquefy the metal is derived from plasma or electric arc. The molten metal is then sprayed in the pattern desired to form a structure by building layers as the metal cools. The resolution achieved by spraying metal is generally poor compared to other processes, to the extent that hybrid machines have been developed to deposit metal, allow it to cool, and then use a milling tool to machine it to size. The speed of the process is slow because sufficient time must be allowed to cool the underlying layer before it can be built upon, as the heat generated by the plasma or electric arc are very high. It is further slowed by the machining process if good resolution is required.
In view of the foregoing, there is a need for improved additive manufacturing techniques for the fabrication of metallic parts that do not utilize metal powders as raw materials, do not generate excessive heat, and do not require time-consuming and uneconomical sintering steps for solidification.